This invention relates generally to toys, games, and other play devices, and more particularly to play devices which exploit the behavior of water on superhydrophobic surfaces.
A hydrophobic substance is one having a distinct tendency to repel water in a manner usually characteristic of non-wetted, oily, waxy or fatty materials. A hydrophobic surface will normally not sustain a water film, even one of monomolecular thickness. This property not only is found in all oils, fats, waxes and many resins, but also in finely divided powders such as carbon black and magnesium carbonate.
A hydrophilic substance has a strong affinity for water by absorption or adsorption even to the point of gradual liquifaction by extracting water vapor from the atmosphere. This property is characteristic of carbohydrates such as algin, vegetable gums, pectins and starches as well as complex proteins like gelatin and albumen.
The present invention deals with a hydrophobic layer formed on a substrate, which layer incorporates particles of hydrophobic fumed silicon dioxide (HFSD). Silicon dioxide particles are produced by the hydrolysis of silicon tetrachloride in a flame process. The fumed silicon dioxide particle is hydrophilic in nature by reason of the large number of hydroxyl groups present on the surface. These particles are rendered hydrophobic by reacting them with a silane. During the reaction, hydrophobic hydrocarbon groups replace many of the hydroxyl groups, the resulting particles offering increased compatibility with organic or non-polar media and a corresponding repulsion to water. One commercially available form of hydrophobic fumed silicon dioxide powder is manufactured and sold by Cabot Corporation of Boston, Mass., under the trademark "Silanox."
Silane, which is a member of the silicone family, contributes its inherent hydrophobicity and oleophilicity to the HFSD particle. Fumed silicon dioxide, which is a fine pure powder, brings to HFSD a particle of extremely small size and enormous surface area, all of it being accessible for interaction with the surrounding media. In protective coatings, the hydrophobicity derived from the silane component of HFSD is augmented by the surface micro-roughness imparted by the silicon dioxide component, giving rise to a degree of water repellency so great that it is often referred to as super-hydrophobicity.
Substrates coated with HFSD repel water to an extraordinary degree. An air layer becomes entrapped between the substrate and the water and is visible as a reflected silvery sheen. This air layer or shield is sometimes referred to as a gaseous plastron.
The super-hydrophobic properties of HFSD can be imparted to substrates in various ways. HFSD particles can, for example, be applied in dry form on tacky surfaces, or it can be applied from a liquid dispersion. But regardless of the mode of applying HFSD to the substrate, it is important that these particles remain essentially uncoated and exposed at the solid-water interface to afford the micro-roughness necessary for optimum super-hydrophobicity.
One serious difficulty often encountered in HFSD coatings is its poor abrasion resistance, for if the coating is subjected to wear, it may be eroded, with a consequent loss of super-hydrophobicity and a possible gain in hydrophilic properties should the underlying substrate be hydrophilic in character.
In applicant's above-identified copending application and patent, there is disclosed a technique for coating substrates to render the face thereof super-hydrophobic, the resultant face being highly abrasion and scratch resistant.
This is accomplished by applying to the face of a substrate which has a micro-rough surface, particles of hydrophobic fumed silicon dioxide dispersed in a solvent within which is dissolved a resinous binder whose amount, by weight, is substantially less than one-half of the amount of particles in the dispersion. Upon volatilization of the solvent, the resultant coating is composed predominantly of fumed silicon dioxide particles strongly bonded to the face of the substrate.